The present disclosure relates to a belt cleaning device and an image forming apparatus with the same.
Conventionally, there have been known image forming apparatuses such as printers employing a tandem developing method in which a toner image of each color is secondarily transferred to a sheet after being primarily transferred onto an intermediate transfer belt. In the tandem developing method, residual toner not transferred to a sheet tends to remain on the belt. Thus, a cleaning unit provided with a fur brush for removing residual toner from the belt is provided on a downstream side in a rotating direction of the belt in some cases. The cleaning unit electrostatically removes toner by applying a bias having a polarity opposite to a charging polarity of the toner to the fur brush (bias cleaning method). However, the residual toner tends to have a polarity opposite to that of the toner at the time of development, i.e. have the same polarity as the fur brush, wherefore it is difficult to electrostatically remove the residual toner. Accordingly, it has been proposed to dispose a bar brush for charging residual toner on a side of the belt upstream of the fur brush.
Recently, inexpensive paper and low-quality paper such as recycled paper have been frequently used as sheets for cost reduction and reutilization of resources. Talc and calcium carbonate are blended in large quantity as a surface treatment agent and a filler in such low-quality paper. Thus, paper powder containing these foreign substances adheres to the bar brush as image formation is repeated.
FIG. 17 is a diagram showing brush bristles 40 of a bar brush 33 in a clean state where no foreign substance adheres to the tips. FIG. 18 is a diagram showing the brush bristles 40 of the bar brush 33 in a state where hardened paper powder adheres to the tips. In a state shown in FIGS. 17 and 18, the tips of the brush bristles 40 are facing toward a front side of the planes of FIGS. 17 and 18. Small and large pieces of paper powder Pa adhere to the tips of the brush bristles 40 shown in FIG. 18. With the brush bristles 40 having the paper powder Pa adhering thereto as shown in FIG. 18, a frictional charging effect for residual toner is reduced and a belt cleaning failure occurs. Further, the paper powder adhering to the brush bristles 40 of the bar brush 33 is hardened and scrapes a belt surface to reduce secondary transferability. As a result, in a conventional image forming apparatus, the quality of an image transferred to a sheet is reduced and it is not possible to obtain images with good quality over a long period of time.
Accordingly, a first method has been proposed in which foreign substances are attracted and collected by insulating threads charged to have a polarity opposite to that of foreign substances. Further, a second method has been proposed in which foreign substances are collected by applying positive and negative biases respectively to two fur brushes.